The present invention relates to an optical sensor for the monitoring of combustion processes in a combustion chamber and to a method for the manufacture of said sensor.
In internal combustion engines, for example spark ignition engines or diesel engines, optical sensors are used to monitor combustion. Recording the brightness over time in a combustion chamber on the one hand provides valuable data useful as boundary conditions for computational models since the temperature can be inferred from the brightness. On the other hand, the soot concentration within the combustion chamber can be deduced by determining the brightness. Due to increasingly harsh regulations with respect to gas emissions legislation a lot of effort is undertaken to decrease the soot formation during combustion in engines. Optical sensors provide valuable data for these developments.
Thus, as described in EP 0593413 for example a plurality of optical sensors are arranged in the cylinder head gasket of an internal combustion engine. For this purpose, the diameter of such sensors must not be larger than about 2 mm. The angular coverage of sensors in this application is 10 to 40°.
A plurality of sensors in the combustion chamber is provided for this field of application to be able to cover the spatial extension of combustion. In this case a correspondingly smaller angular coverage is provided.
Other fields of application provide a single sensor which is intended to collect as much as possible of the light emitted in the combustion chamber. A large angular coverage is required from such a sensor. Sensors used for this purpose have an angular coverage of about 110°. The diameter of such sensors is more than 10 mm. Therefore, such sensors cannot be built into a spark plug or heater plug.
These sensors are for example mounted directly into the cylinder head in openings provided for this purpose. Difficulties arise during mounting and sealing the resulting openings.
Another possibility is to mount an optical sensor into a spark plug of a spark ignition engine or into a heater plug of a diesel engine. This is for example achieved by means of a thread provided on the sensor. This provides the advantage of rapid assembly and disassembly and does not require additional mechanical work on the engine.
In general, optical sensors are made of a lens which lets pass the light impinging onto one surface and emits it from the opposite surface. Subsequently, the light impinges on a waveguide which conducts the light to a monitoring device. At this monitoring device, the light signal is processed into data and edited in a form which enables further evaluation of the information.
The lens as well as one end of the waveguide are usually surrounded and kept together by a sheath. The difficulties in centering are mainly encountered in the case of very small diameters and depths of the lenses.